1. Field of the Invention
The present invention relates to an improved one-step process for the production of elastomeric molded articles having a closed surface layer by reaction injection molding of highly reactive systems of polyisocyanates, relatively high molecular weight polyhydroxyl(polyamino) compounds and polyamines; the mixtures of isocyanate reactive compounds suitable therefor; and the molded articles obtained by the process.
2. Description of the Prior Art
The production of elastomeric molded articles having a closed surface layer by reaction injection molding is known (see e.g. DE-AS No. 2,622,951 (U.S. Pat. No. 4,218,543), EP-A- No. 00 81 701, U.S. Pat. No. 4,296,212, U.S. Pat. No. 4,324,867 or U.S. Pat. No. 4,374,210).
With suitable choice of the starting components it is possible by this process to produce both elastomeric and rigid products and any variations in between.
The resulting molded articles are used, for example, as shoe soles or in particular as car body parts in the automobile industry. Processing of the raw materials is carried out by the reaction injection molding process (RIM process). This is a technique in which highly reactive, liquid starting components are injected within a very short time into a mold by high pressure dosing apparatus with high output after they have been mixed in force controlled mixing heads. It is described, for example, in DE-AS No. 2,622,951 how even extremely reactive systems, i.e. one-shot mixtures of di- or polyisocyanates based on 4,4'-diisocyanato-diphenylmethane which are liquid at room temperature, aromatic polyamines, relatively high molecular weight polyhydroxyl compounds containing primary hydroxyl groups and powerful catalysts can be worked up with cream times of less than one second.
Solidification of the reaction mixture after injection takes place so rapidly that in the case of extremely reactive mixtures the mold can be opened after only 5 seconds to remove the part from the mold.
One-shot mixtures of a polyisocyanate component, aromatic diamines and polyethers containing primary and/or secondary amino groups result in polyurea elastomers when worked up by the RIM process (see e.g. EP-A- No. 0,081,701).
Polyurea elastomers which are to be used as material for car body elements are required to have a high rigidity. By using glycols as co-chain lengthening agents it is possible to increase the flexural strength of the molded parts, but the thermal properties such as the flexural modulus at high and low temperature and the stability under heat (sag) are then adversely altered.
There are also limits to the amount of diamine chain lengthening agent which may be used in a one-shot system. Under normal processing conditions (50.degree.-70.degree. C. mold temperature), the molded parts produced are so brittle that they cannot be removed intact even after a prolonged dwell time in the mold. Thus, for example, in materials prepared from about 23 to 25% by weight of diethyltoluylenediamine (DETDA), an increase in the flexural modulus which is generally from about 350 MPa (when relatively high molecular weight polyhydroxyl compounds are used) to about 550 MPa (when polyethers with aromaticlly bound amino groups are used as relatively high molecular weight reactants for the polyisocyanates), is not possible without the use of fillers. When fillers are included, in particular glass fiber fillers, a flexural modulus of about 1000 to 1500 MPa can be obtained when using similar amounts of DETDA.
The process of DE-OS No. 3,520,326 constitutes a major advantage when compared with the known art process discussed above in that by increasing the mold temperature from the usual range of about 50.degree.-70.degree. C. to at least 105.degree. C. it enables diamine chain lengthening agents to be used at a higher concentration while still enabling the molded articles to be removed from the mold, with the result that a substantial increase in the flexural modulus may be obtained. This process has the disadvantage, however, that it must be carried out at quite uncustomarily high molding temperatures. At these high molding temperatures the internal mold release agents generally used, in particular those based on combinations of zinc carboxylates such as zinc stearate and aliphatic polyamines, to a large extent lose their effect.
It was therefore an object of the present invention to provide an improved process for the production of elastomeric molded articles having a closed surface layer by reaction injection molding using the above-mentioned starting materials, which process combines the following advantages:
The process should enable molded articles to be produced with a flexural modulus distinctly above 400 MPa at room temperature.
The molded articles should be readily removable from the mold (i.e., adequate "green strength").
Internal mold release agents should be able to be used in the process without entirely or partly losing their effect as in the case of high molding temperatures.
This problem was able to be solved by the process according to the invention described below. It was surprisingly found that molded articles satisfying the requirements mentioned above could be produced if the reaction mixture contained certain reaction products of polyepoxides and aromatic diamines described in more detail below.